A common sampling technique for determining the chemical analysis of molten metal utilizes a measurement of the temperature at which the molten metal solidifies. Accordingly, a sampling device having an internal cavity and a temperature sensing element positioned within the cavity is immersed into a bath of the molten metal, the molten metal enters the cavity through an inlet, and the cavity fills under the force of ferrostatic pressure. In typical prior art sampling devices, the molten metal sample solidifies along a solidification front that progresses from the walls of the cavity toward the thermal and geometric center of the cavity.
As is known, during solidification, gases in the molten metal become segregated along the solidification front and may accumulate to form gas voids in the last metal to freeze, i.e. near the thermal center of the cavity. A problem arises with typical prior art sampling devices, however, in that the temperature sensing element is generally positioned at or near the center of the filled cavity and the adjacent gas voids interfere in the heat transfer from the solidifying metal to the sensing element. The interfered heat transfer results in inaccurate and inconsistent temperature sensing measurements. A need exists, then, for a sampling device that controls molten metal solidification to prevent gas voids from being formed at or near the temperature sensing element.
Another problem with a typical prior art sampling device is that the reliability of the device is dependent upon the ability of the internal cavity to fill completely when the device is immersed within the molten metal bath and to remain filled when the device is withdrawn from the molten metal bath. As may be understood, void formation near the temperature sensing element can also occur when a portion of the molten metal sample within the cavity escapes and/or when the cavity does not completely fill with the molten metal. A need exists, then, for a sampling device having an internal cavity that substantially completely fills when the sampling device is immersed within the bath and that remains substantially completely filled when the sampling device is removed from the bath.
In prior art sampling devices, it is also known to have two separate temperature sensors, one for measuring the liquidus temperature at which the molten metal solidifies and one for measuring the ambient bath temperature of the molten metal bath. In the case where the temperature sensing devices are thermocouples, it is necessary to maintain a constant temperature for the junctions of the thermocouples and their respective compensated lead wires. However, still another problem with a typical prior art sampling device is that latent heat given off by the solidifying metal in the internal cavity can affect such junctions and compensated lead wires and may result in inaccurate temperature readings. A need exists, then, for a sampling device that shields such junctions and lead wires from such latent heat.